In computer-assisted knee arthroplasty, a cutting block is commonly used that includes a cutting guide to which a localization reference, such as an infrared radiation emitting or reflecting reference star, is attached. The localization reference can be spatially adjusted (for example, rotated) to allow a medical navigation system to have a good view of the reference. Using the localization reference and the medical navigation system, the cutting block may be positioned such that the cutting guide is located in a planned incision plane.
EP 1 690 503 A1 discloses a bone cutting block that includes: a cutting guide to which a localization reference is adjustably attached that allows the incision plane of the cutting guide to be spatially determined; a fixation device that can be fixed to a bone; an adjusting device between the fixation device and the cutting guide, wherein the incision plane of the cutting guide relative to the bone can be set using the adjusting device, wherein the spatial location of the adjusting device can be determined using a registration element. EP 1 690 503 A1 is incorporated by reference herein in its entirety.
Such adjusting devices may include setting elements such as hand wheel screw setting elements. Using the setting elements, the cutting guide can be rotated about at least two non-parallel axes and adjusted in height, wherein each setting element may change one mechanical degree of freedom of the cutting guide. The change in a mechanical degree of freedom of the cutting guide leads to a change in the corresponding degree of freedom of the incision plane with respect to a bone coordinate system (with respect to which the cutting guide or the incision plane of the cutting guide is defined). Such a conventional cutting block enables navigation with respect to a predetermined navigation plan. In the navigation plan, an incision plane may be set or aligned to a predetermined spatial location of a target incision plane using a medical navigation system.
Use of a conventional cutting block requires a time-consuming number of iterations by repeatedly adjusting the setting elements. In so-called “navigation-to-reference,” the incision plane is aligned in a reference coordinate system determined by defined projection planes. The projection planes can be defined with respect to pre-operatively determined references such as characteristic planes or axes or points of a body. The reason that a number of iterations are required is that changes made to a setting element of the adjusting device not only cause a change in the desired degree of freedom of the incision plane in the reference coordinate system, but additionally changes at least one other degree of freedom of the incision plane relative to the references. If, for example, particular distances of the incision plane of the cutting block relative to planes or axes or points of the body are to be maintained, and if a first distance has been set by repeatedly adjusting the setting elements, the attempt to set a second distance of the incision plane or cutting guide relative to another axis or plane or point by changing the respective setting element will also adjust the first distance. Therefore, a large number of iterations are often required to precisely set the incision plane with respect to the references in the reference coordinate system.
Navigation with respect to references or reference objects of a body means that the incision plane of a cutting guide can be set in a reference coordinate system with respect to characteristic (or pre-operatively or intra-operatively determined or selected) reference objects of a body. Such reference objects of the body may include:                characteristic planes (for example, the tibial plateau, the sagittal plane, the frontal plane or the transversal plane);        axes (for example, the femoral axis or the tibial axis); or        points of a body (for example, the attachments of the cruciate ligaments).The incision plane can be set such that it maintains specified distances from the planes, axes, or points of the body that are predetermined by the surgeon.        
This method of navigation ordinarily benefits from the experience of the surgeon, who knows the distances that the cutting guide or the incision plane of the cutting guide should maintain relative to the selected references for the cutting guide to be properly placed. Some surgeons prefer to navigate with respect to reference objects, since the distances from the reference objects that are desired can be directly set. Additionally, navigation with respect to reference objects eliminates the need for a navigation planning step.